In radio systems, it is often desirable to combine a number of RF signals so that they may be transmitted from a single antenna. One reason why this might be desirable, for example, is that the transmitter site might not be large enough to accommodate separate antennas for each RF signal. Another reason why this might be desirable is that there might be a large number of RF signals and it is simply more cost-effective to combine them onto a single antenna feed.
In order to combine a number of RF signals from radio transmitters and couple them to a common antenna, each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. Various prior RF signal combiner arrangements have been known for combining a multiplicity of RF signals for transmission by a single antenna. One such prior arrangement is the subject of U.S. Pat. No. 4,375,622, dated Mar. 1, 1983, entitled "Multiport Radio Frequency Signal Combiner," by joint inventors Allen H. Hollingsworth (the inventor of the present invention) and Alan G. Deutschle, and assigned to Motorola, Inc. (the assignee of the present application).
While prior combiners have been adequate for combining multiple transmitter outputs for a single antenna, however, the inventor is unaware of any prior combiner that is suitable for combining outputs of transmitters onto antennas where the transmitters may be operated at more than one frequency. This type of combiner would be advantageous, for example, for a trunked or cellular system application in which subscriber loading at a particular site may be sufficiently light so that a full complement of repeaters may not be warranted.